1. Field of the Invention
This invention relates generally to bicycle frame rear suspension systems, and more particularly to a bicycle frame with rear passive suspension configured to elastically deflect predominantly in a vertical direction under normal cycling loads so as to better isolate the rider from road-surface irregularities and roughness, while simultaneously maintaining or improving rigidity in other directions so as to preserve or improve handling.
2. Brief Description of the Prior Art
Many prior art bicycle frames with active suspension designs (those which use dedicated and purpose-built devices for flexibility and/or vibration control) often incorporate a spring to soften the ride. Active suspensions, although common on off-road mountain bicycles, typically add too much weight and reduce power transmission under hard pedaling for high-performance road bicycles and other applications where weight, simplicity, and pedaling efficiency are critical.
Many prior art bicycle frames with passive suspension designs rely on the flexibility of curved seat stays to soften the ride. However, the amount of flexibility in curved seat stays is typically quite limited (almost insignificant by comparison to the compressive flexibility of pneumatic rear tires) because of high material stresses, the possibility of bucking slender members under compression, and also because the right and left seat stays can flex at different rates depending on direction of wheel load. If curved seat stays are designed and fabricated to be sufficiently elastic as to permit a significant amount of rear wheel travel, one side of the rear wheel's axle would travel vertically a greater distance than the other side when the rider leans the bicycle from side to side as when sprinting or climbing (because this rider-induced action creates transverse loads applied at the bottom of the tires where they contact the road). This transverse load at the rear tire would flex the rear curved seat stays at different rates throwing the rear wheel out of alignment with the bicycle's frame; which would make a bicycle inefficient and hard to control when pedaling out of the saddle while climbing or sprinting.
Some recently developed bicycle frame designs use carbon fiber seat stays in place of metal to soften the ride, but are too stiff in compression to adequately flex to isolate the rider from vibration other than those of very low amplitude.
Another recently developed frame design, known as a “Beam Bicycle”, relies on a flexible cantilever beam to suspend the rider's saddle. These designs are effective in isolating the saddle, but do not isolate the bottom bracket and hence the pedals, thus allowing much of the road vibration to reach the rider's legs. Additionally, these frames are typically heavier because they must resist the high bending forces created by cantilevering much of the rider's weight from near the head tube of the frame. Another disadvantage of this design is that the cantilever suspension system depends on varying the distance between the saddle and the bottom bracket, thus the pedaling forces promote rider bounce, which is inefficient.
Nicol, U.S. Pat. No. 3,966,230 discloses a bicycle frame formed of lightweight, high strength relatively large diameter metal tubing, such as titanium, provided with integral stiffeners at the top of the seat tube, the bottom of the down tube and the top of the fork blades. In larger sizes of frames, stiffeners may also be included at the bottom of the seat tube and at the top of the down tube. The placement of stiffeners at these selected locations results in overall frame stiffness characteristics not previously considered achievable without substantial increase in the weight of the frame.
Klein, U.S. Pat. No. 4,621,827 discloses a lightweight bicycle with two chainstay tubes made of tubing of unequal rigidity, specifically the chainstay tube on the chain or drive side is made heavier, while the other chainstay tube is lightened, in order to increase power train efficiency by reducing the magnitude of frame deflection caused by chain stress. In the preferred embodiment, the metal that is added to the chain side chainstay tube is taken from the non-drive side so that there is no net addition of weight
Kerr, U.S. Pat. No. 5,240,269 discloses a bicycle suspension device employed between the rear wheel and the frame of a mountain bike. A lever arm pivoted to the frame carries the rear wheel at one end and the suspension device at the other. The rear wheel portion of the lever arm is about four times as long as is the suspension device portion. The suspension device includes a hollow elongated elastomeric spring carrying an aluminum telescoping portion. The telescoping portion supports the elastomer while allowing reciprocating movement along a major axis, but not allowing any rotational movement around that same axis.
Klein et al, U.S. Pat. No. 5,586,780 discloses an improved bicycle suspension system including a rotary damper and composite springs which closely control the direction of allowable movement in the suspension to preclude rocking, twisting or unwanted lateral deflections.
Chou, U.S. Pat. No. 5,593,168 discloses a bicycle frame with shock absorbing system that eliminates the shocking force occurring on the connecting portion of a seat tube fixedly mounted on a bottom bracket and a down tube. The system includes a shock absorber mounted between the seat tube and the down tube, a pair of brackets integrally formed with bottom bracket extend toward the down tube. A pivot is fixedly secured to the brackets and a pivot mounting tube is integrally formed on a rear end of the down tube and pivotally mounted on the pivot.
Kirk, U.S. Pat. No. 6,109,637 discloses a bicycle frame that uses existing seat stay members as a spring, and absorbs high frequency, low amplitude vibration with effective damping. Each seat stay is connected at a top end to the seat tube, and pivotally connected at the bottom end to the rear drop out, and has a circular arcuate curvature adjacent the bottom end with a radius of between about 11-16 inches and subtending an angle of between about 20-60 degrees. A damping/spring constant increasing element, comprising a metal strip bonded to elastomeric material (such as silicone) having a durometer between about 20-70 Shore A, is bolted to the seat stay convex arcuate portion, and can be replaced with a unit having a different durometer to adjust the spring force constant for rider weight, riding style, road or terrain conditions, or the like.
The present invention is distinguished over the prior art in general, and these patents in particular by a bicycle frame having a rear passive suspension system configured to elastically deflect predominantly in a vertical direction under normal cycling loads so as to better isolate the rider from road-surface irregularities and roughness, while simultaneously maintaining or improving rigidity in other directions so as to preserve or improve handling. The frame has a pair of upper chain stays that control the rear wheel motion rigidly in five of the six degrees of freedom with stiffness comparable to existing road bicycle designs, while simultaneously allowing deflection of the tubular members and a significant amount of vertical movement of the rear wheel for ride compliance in order to improve handling, comfort, and safety.